Surface fouling of metallic surfaces used in equipment for fuel or steel production systems, such as systems for manufacturing coke or coke byproducts, is a significant problem. Historically, recapture of coke oven byproducts during coke manufacturing was an economically important element of the coke manufacturing process because the byproducts were useful in other applications in agriculture and in the chemical industry. Today, the main emphasis of modern coke byproduct plants is treating the coke oven gas sufficiently so that it can be used as a clean, environmentally friendly fuel. As such, efficient systems and methods for coke production are highly desirable.
As coal is converted into coke, volatile matter in the coal is vaporized and driven off. This volatile matter leaves the coke oven chambers as hot, raw coke oven gas. Raw coke oven gas typically contains a variety of organic and inorganic contaminants including tar, light oils (BTX), naphthalene, ammonia, hydrogen sulfide and hydrogen cyanide. In order to make raw coke oven gas suitable as a fuel gas, a series of processes is typically performed including cooling the coke oven gas to condense out water vapor and contaminants, removing tar to prevent or reduce gas line and equipment fouling, removing ammonia to prevent gas line corrosion, and removing naphthalene to prevent gas line fouling by condensation. Generally, these processes involve gas and liquid treatment.
Gas treatment in coke manufacturing includes some basic processes and equipment for achieving the desired processing of the raw coke oven gas and the associated processing of additional product streams constituting one or more materials removed from the coke oven gas. Treating raw coke oven gas typically involves cooling the gas to remove water vapor and reduce its volume. As the coke oven gas is cooled, a portion of the higher boiling components of the stream including, for example, water, tar and naphthalene condense and are removed from the gas stream. This condensate collects in the primary cooler system and can be a source of surface contamination or fouling on the heat exchanger tubes and other downstream equipment surfaces. Additionally, contaminants such as tar vapor condense and form aerosols that are carried along with the gas flow and ultimately foul downstream surfaces. In the case of heat exchanger surfaces, reduction of heat transfer efficiency results in undesirable downtime and maintenance expense in order to remove the fouling deposits.
Other gas treatment processes include ammonia removal through ammonium sulfate crystallization, ammonia scrubbing and absorption. Further, a final cooler is sometimes used for removing the heat of compression from the coke oven gas that is added as the gas flows through the exhauster. During the final cooler operation, when the coke oven gas is cooled below the outlet temperature of the primary cooler, naphthalene will condense from the gas. This naphthalene readily crystallizes out from the cooling medium and can foul equipment. Sometimes wash oil is applied in the final coolers to dissolve the naphthalene, and a side stream of oil is steam-stripped to remove the naphthalene. Light oils may be removed in a similar fashion.
Liquid treatment processes use a flushing liquor that circulates between the byproduct plant and the coke oven battery. The tar and liquor plant may also process waste water generated by the coke making process and resulting from coal moisture and chemically bound water in the coal. The flushing liquor flows into tar decanters where the tar separates out from the water and is pumped to storage for later sale. Heavier solid particles separate out from the tar layer and are removed as tar decanter sludge. The aqueous liquor may be then pumped back to the battery, with a portion bled off from the circuit as “excess liquor” or waste water. This liquid contains ammonia and, after the further removal of tar particles, it is steam stripped in a still.
The above-described gas and liquid treatment processes lead to surface fouling of equipment that reduces system efficiency by negatively impacting system uptime and equipment longevity.